Combustion control method in vehicle engine and engine system for vehicle

ABSTRACT

A combustion control method during a rich spike engine operation includes the steps of: specifying an exhaust gas A/F based on a first output from a NOx sensor on an upstream side of a catalyst; specifying a NOx concentration in the exhaust gas on the upstream side of the catalyst based on a second output from the NOx sensor; and when the A/F is less than a reference range and the NOx concentration is not more than a reference value, calculating a value P1−P0 between a cylinder pressure P1 at EVO during the rich spike operation and that pressure P0 during a normal operation, when P1−P0 is larger than the reference range and P1&gt;P0, a fuel injection timing at a post injection is retarded, when P1−P0 is smaller than the reference range and P1&lt;P0, the timing is advanced.

CROSS-REFERENCE TO RELATED APPLICATION

The present application claims priority from Japanese application JP2018-003632, filed on Jan. 12, 2018, the contents of which is herebyincorporated by reference into this application.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to combustion control in a vehicle engine,and more particularly to control based on a NOx concentration in exhaustgas.

Description of the Background Art

In recent years, automobiles equipped with cylinder pressure sensorshave become widespread. In such an automobile, the combustion mode of anengine rotating at high speed can be grasped for each crank angle bymeasuring the pressure inside a piston with a cylinder pressure sensorattached to the upper part of the piston of the engine is known (see,WO2011/117973). The combustion mode can be grasped on board with thecylinder pressure sensor; therefore it is deemed to be one of thequickest and the high responsive sensor for any sensor involvedparticularly in engine control among the known sensors. Technically, itis also possible to grasp the cylinder temperature by assuming themomentary change of the output value from the cylinder pressure sensoras polytropic change.

Meanwhile, NOx from automobiles, which has been regarded as an airpollutant in recent years, is usually generated by combusting nitrogenas inert gas, in a high temperature field. Also, there is a relationshipbetween the oxygen concentration in the engine cylinder (which can becalculated from the intake oxygen concentration), the cylindertemperature, and NOx generation. Therefore, by holding informationindicating the relationship in advance in the engine ECU as a map orfunction, estimation of an amount of NOx discharged from the engine canbe performed on board and in real time based on temperature informationobtaining from an output from the cylinder pressure sensor, intake airvolume, and a cylinder oxygen concentration (see, Japanese PatentApplication Laid-Open No. 2009-287410). In such a case, the intake airvolume and the cylinder oxygen concentration are calculated based on acylinder pressure or an output of an air flow sensor, an opening area ofan exhaust gas recirculation (EGR) valve constituting an EGR device inthe engine, an output from a pressure sensor equipped with an EGRsystem, and an output value from an oxygen sensor equipped with an inletsystem or an exhaust system.

The combustion control of the engine based on the output value from thecylinder pressure sensor has the following problems.

First of all, the cylinder pressure sensor is remarkably highlyresponsive, however, a dedicated ECU for high speed calculation is alsorequired in addition to being expensive per se. In addition, dependingon a type of vehicle and destination of the automobile, a plurality ofcylinder pressure sensors could be required in some cases. Therefore, itis disadvantageous in terms of cost and securing of mounting spacetherein.

Further, the cylinder pressure sensor is highly responsive, therefore,the amount of data output is also large, however, the calculation speedof the engine control ECU to which the dedicated ECU is connected cannotcatch up with the output. Therefore, when the cylinder pressure sensoris actually used, many of the output data are thinned out, or some ofthe output data are supplied to calculation of engine control logic.Therefore, it is also disadvantageous from the viewpoint of costeffectiveness.

Further, the cylinder pressure may instantaneously swing to the highpressure side or the low pressure side because of the reaction forceoriginating from steps on the road surface, which transmits from tiresthrough a drive train and acts in the direction opposite to the enginerotation direction. Therefore, when the engine control is operated basedon the output value of the cylinder pressure sensor, the calculation inthe engine control ECU is possibly affected by pressure fluctuationattributed to a factor other than the combustion in the engine.Particularly, when data processing such as thinning out the output datafrom the cylinder pressure sensor is adopted, it is likely to besusceptible to such influence.

The inventors of the present invention has built a hypothesis in whichwhen the amount of NOx discharged from the engine is equivalent, thecombustion history in the engine cylinder should be equivalent throughthe assiduous study and achieved to conceive a new technique ofcombustion control of engine based on the NOx amount in the exhaust gasinstead of combustion control based on the output value from thecylinder pressure sensor.

Also, in an exhaust path in the engine, an occlusion-type NOx reductioncatalyst (LNT) for occluding NOx is generally provided for reducing NOxin the exhaust gas. The LNT occludes NOx during the engine is in normaloperation, and at a timing when it is determined or predicted that theocclusion amount approaches to the upper limit, the NOx purge isperformed in which a short-period fuel injection (rich spike) isperformed as post injection, such that the occluded NOx is reduced to N₂with the fuel injected as a reducing agent.

In such a case, the NOx purge execution timing may be determined, inaddition to based on an estimated occlusion NOx amount estimated by timeintegration of estimation values of NOx amount in the exhaust gas, basedon a NOx amount measured by a NOx sensor. However, control is executedon the safe side; therefore, there is a tendency that the frequency ofrich spike becomes high, leading to such problems as deterioration offuel economy, inflow of hydrocarbon (HC) and carbon monoxide (CO) on thedownstream side, and deterioration of driveability caused by torque.

Although it may be considered to control the rich spike operation basedon the monitoring result of the oxygen concentration in the exhaust gasbased on the output from the oxygen sensor provided on the downstreamside of the LNT, the timing of detecting the enrichment is late;therefore there are problems in terms of real-time property and fueleconomy. Further, even if the A/F is maintained in the adaptation statebased on the monitoring result, it is not necessarily guaranteed thatNOx is actually reduced appropriately.

The inventors of the present invention have also found that thecombustion control method of the engine based on the above-described NOxamount can be applied even for NOx purge by post injection, as a resultof assiduous study.

SUMMARY

The present invention relates to combustion control for a vehicleengine, and more particularly to control based on a NOx concentration inan exhaust gas.

According to the present invention, a combustion control method in anengine of a vehicle during a rich spike operation, in which the engineincludes a fuel injection element attached to, and an exhaust path foran exhaust gas discharged from the engine includes a NOx occlusioncatalyst and a first NOx sensor provided on an upstream side of the NOxocclusion catalyst, includes the steps of a) performing a rich spikeoperation in the engine, b) specifying an air-fuel ratio of the exhaustgas discharged from the engine during the rich spike operation based ona first output value output from the first NOx sensor in accordance withan oxygen concentration in the exhaust gas, c) specifying a NOxconcentration in the exhaust gas on the upstream side of the NOxocclusion catalyst based on a second output value output from the firstNOx sensor in accordance with a NOx concentration in the exhaust gas,and d) controlling a combustion condition of the engine during the richspike operation. In the step d), a difference value P1−P0 between acylinder pressure P1 at an exhaust valve opening timing in the engineduring the rich spike operation and a cylinder pressure P0 at an exhaustvalve closing timing in the engine during a normal operation iscalculated, when the air-fuel ratio is less than a predeterminedreference range previously determined based on a stoichiometric air-fuelratio and the NOx concentration exceeds a predetermined reference value,and in the case that the difference value P1−P0 does not satisfy thepredetermined reference range and P1>P0, a fuel injection timing at apost injection from the fuel injection element to the engine isretarted, and in the case that the difference value P1−P0 does notsatisfy the predetermined reference range and P1<P0, the fuel injectiontiming at the post injection from the fuel injection element to theengine is advanced.

According to the method, the rich spike operation control is performedin terms of real-time property and fuel economy, as compared with thecase where control based on the output from the oxygen sensor providedon the downstream side of the NOx occlusion catalyst is performed.

Accordingly, it is an object of the present invention to provide a novelcombustion control method during a rich spike operation of a vehicleengine.

These and other objects, features, aspects and advantages of the presentinvention will become more apparent from the following detaileddescription of the present invention when taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B are graphs illustrating fundamental conceptions ofequal-NOx combustion control;

FIG. 2 is a diagram illustrating a structure for intake and exhaust ofan engine system 1000;

FIG. 3 is a flowchart illustrating specific processing procedure of theequal-NOx combustion control;

FIG. 4 is a flowchart illustrating a procedure of a combustion statecalculation performed in the equal-NOx combustion control;

FIG. 5 is a graph illustrating a chronological change in a relationshipbetween the NOx concentration of exhaust gas EG from an engine main body100 and the temperature in the engine from the start of combustion;

FIGS. 6A, 6B, and 6C are graphs illustrating crank angle dependency of acylinder temperature, a cylinder pressure, and a heat generation rate inthe engine main body 100 in the case of diffusion combustion;

FIG. 7 is a flowchart illustrating a specific processing procedure ofequal-NOx combustion control during rich spike operation;

FIGS. 8A, 8B, and 8C are graphs illustrating crank angle dependency of acylinder temperature, a cylinder pressure, and a heat generation rate inthe engine main body 100 in the case of the rich spike operation; and

FIG. 9 is a flowchart illustrating of a modification of the processingprocedure of equal-NOx combustion control; and

FIG. 10 is a schematic cross-sectional view illustrating a configurationexample of a NOx sensor NS.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

<Fundamental Conception of Equal-NOx Combustion Control>

First, the fundamental conception of equal-NOx combustion control (alsoreferred to as equal-NOx control), which is a combustion control methodfor a vehicle engine according to the present embodiment, will bedescribed. The equal-NOx combustion control is a control method formaintaining the combustion state in the engine in a steady state.Briefly, the NOx concentration in the exhaust gas discharged from theinternal combustion engine is measured, and the operation of the engineis controlled such that the NOx concentration is kept substantiallyconstant, whereby the state of the combustion in the engine ismaintained to be the steady state.

FIGS. 1A and 1B are graphs illustrating fundamental conceptions of theequal-NOx combustion control performed in the present embodiment.Specifically, in FIG. 1A and FIG. 1B, crank angle (corresponding totime) dependency of the heat generation rate in the engine (heatgeneration rate profile) and fuel injection pulses for defining fuelinjection timing before and after the equal-NOx combustion control inthe case of partial premix combustion as an example are illustratedtogether.

Indicated by a broken line in FIGS. 1A and 1B is an ideal heatgeneration rate profile (ideal profile) pf0 corresponding to a certainfuel injection pulse ip1. As the ideal profile pf0, it is preferable tohave a waveform such that the peak is located on the retard angle sideby about 5° C.A from the top dead center TDC of the piston. Although,the best fuel efficiency is achieved when the combustion peak coincideswith the TDC, in that case, the noise is considerably increased;therefore, it is generally considered that the slightly retarded angleis preferable.

However, when the fuel injection pulse ip1 is actually given, the heatgeneration rate changes along the heat generation rate profile pf1 asindicated by the solid line in FIG. 1A, which is later in time than theideal profile pf0, or along the heat generation rate profile pf2 asindicated by the solid line in FIG. 1B, which is earlier in time thanthe ideal profile pf0 in some cases. The former corresponds to a casewhere the ignition timing is later than the ideal for some reason andthe latter, similarly, corresponds to the case where the ignition timingis earlier than the ideal.

In the equal-NOx combustion control performed in the present embodiment,such a deviation of the ignition timing from the ideal state is detectedbased on a change in the concentration of NOx discharged from the engineas a trigger. When it is determined that the ignition timing is behindthe ignition timing in the ideal state as illustrated in FIG. 1A, theignition timing is advanced, and as illustrated in FIG. 1B, when it isdetermined that the ignition timing is earlier than the ignition timingin the ideal state, the ignition timing is retarded. As a result, asillustrated on the right side in FIGS. 1A and B, the heat generationrate profile pf1 or pf2 substantially coincides with the ideal profilepf0, that is, the combustion state of the engine close to the ideal isrealized.

Meanwhile, in some cases, despite that the ignition timing is equal tothe ideal state, it is determined that the cylinder pressure(in-cylinder pressure) in the engine is excessive or too small comparedto the reference value. In such a case, by adjusting the boost pressureby the turbo (turbocharger) or the recirculation amount (EGR amount) ofthe exhaust at the EGR, a heat generation rate profile close to theideal profile pf0 is obtained.

Such equal-NOx combustion control is based on the assumption that thehypothesis that a cylinder combustion history in the engine isequivalent if the NOx concentration in the exhaust gas is equivalent.

It should be noted that the combustion waveforms exemplified in FIGS. 1Aand 1B exemplify a premixed combustion mode that generates heat laterthan the main injection timing. However, the above description holdstrue in the case of diffusion combustion, which is a standard combustionmode of a diesel engine in which combustion starts during maininjection.

Configuration Example of the Engine System

FIG. 2 is a diagram illustrating a structure for intake and exhaust ofan engine system 1000 for a vehicle which is one mode of a controltarget of the equal-NOx combustion control according to the presentembodiment.

The engine system 1000 for a vehicle illustrated in FIG. 2 is afour-cylinder diesel engine system including four cylinders (morespecifically, combustion chambers) 101 in an engine main body 100mounted on a vehicle (not shown). In the present specification, theengine system 1000 for a vehicle and the engine main body 100 aresometimes simply referred to as engines without distinction. As theengine main body 100, a known configuration (so-called 4 cycle engineconfiguration) is used. Therefore, the detailed illustration anddescription of the components provided in the engine main body 100 suchas a piston, a crankshaft, an intake valve, an exhaust valve, a fuelinjection device, and a fuel injector will be omitted. The operation ofeach part of the engine system 1000 for a vehicle is controlled byexecution of a predetermined driving control program in an electroniccontrol unit (ECU) 200 storing thereof in advance.

In the engine system 1000, briefly, intake gas (air) IG taken from anintake port 1 a and reaching the engine main body 100 via an intake path1 is used for combustion. On the other hand, the exhaust gas EG from theengine main body 100 is discharged outside through an exhaust path 2through an exhaust port 2 a.

A variable capacity type turbo (VN turbo) 11, which is a type ofturbocharger and controls the supply pressure of the intake gas IG byusing the exhaust gas EG from the exhaust path 2, is provided in therespective intake path 1 and exhaust path 2. A water-cooled intercooler12 is provided downstream of the VN turbo 11 in the intake path

On the other hand, on the downstream side of the VN turbo 11 in theexhaust path 2, an occlusion-type NOx reduction catalyst (LNT) 13 foroccluding NOx in the exhaust gas EG and a diesel particulate removaldevice (DPF) 14 for removing particulate matter (PM) in the exhaust gasEG are provided in this order, and furthermore, on the downstream sidethereof, a selective catalytic reduction denitrification device (SCR) 15for decomposing NOx by urea is provided together with a urea supplysource 16 for supplying urea to the SCR 15.

A branch path 3 branches off from the exhaust path 2 between the enginemain body 100 and the VN turbo 11, and is connected to the intake path 1on the downstream side of the intercooler 12. Similarly, a branch path 4branches off from the exhaust path 2 between the DPF 14 and the SCR 15,and is connected to the intake path 1 on the upstream side of the VNturbo 11. An EGR cooler 17 is provided in the middle of the branch path4.

The branch paths 3 and 4 allow part of the exhaust gas EG from theengine to be mixed into the intake gas IG. That is, the part of theexhaust gas EG is circulated (recirculated) without being discarded andsucked again. In FIG. 2, the gas supplied to the engine main body 100 iscollectively referred to as intake gas IG which is categorized into thefollowing manner that intake gas IG taken in from the intake port 1 a isparticularly referred to as intake gas IG1, and the gas in which theintake gas IG1 is mixed with the exhaust gas EG passing through thebranch path 4 is particularly referred to as intake gas IG2, and the gasin which the intake gas IG2 is mixed with the exhaust gas EG passingthrough the branch path 3 is particularly referred to as intake gas IG3.

In the intake path 1, a throttle ST1 is provided on the upstream side ofa junction with the branch path 4, and the throttle ST2 is provided onthe upstream side of a junction with the branch path 3. Meanwhile, avalve VLV1 is provided in the middle of the branch path 3, and a valveVLV2 is provided in the middle of the branch path 4. The supply ratio ofthe exhaust gas EG to the intake gas IG is controlled by the throttleST1, ST2 and the valves VLV1, VLV2. A configuration for controlling thesupply amount of the exhaust gas EG in the intake gas IG3 by controllingthe opening and closing of the valve VLV1 is referred to as a highpressure exhaust circulation device EGR1 and a configuration forcontrolling the supply amount of the exhaust gas EG in the intake gasIG2 by controlling the opening and closing of the valve VLV2 is referredto as a low pressure exhaust circulation device EGR2.

Further, in the engine system 1000, various sensors are provided invarious places. Specifically, an airflow sensor 21 for detecting theflow rate of the intake gas IG is provided upstream of the throttle ST1in the intake path 1, and the VN turbo 11 is provided with a rotationspeed sensor 22 for detecting the rotation speed of the turbine. Also, ahumidity sensor 23 is provided on the downstream side of the intercooler12 in the intake path 1. Further, an intake air pressure sensor 24 andan intake air temperature sensor 25 are provided in the engine main body100.

On the other hand, in the exhaust path 2, the NOx sensors NS (NS1, NS2,NS3) are provided, specifically, each sensor is provided near theupstream side of the LNT 13, near the upstream side of the SCR 15 (ofthe urea supply source 16) and near the downstream side of the SCR 15.In addition, an oxygen sensor OS is provided near the downstream side ofthe DPF 14. It should be noted that, for the NOx sensor NS1 providednear the upstream side of the LNT 13 at least, a sensor which isprovided so that the oxygen concentration can be measured is used.Temperature sensors 27, 28, and 29 are provided near the upstream sideof LNT 13, near the downstream side of LNT 13 (between LNT 13 and DPF14), and near the downstream side of DPF 14, respectively. In addition,an exhaust pressure sensor (differential pressure sensor) 30 fordetecting the pressure difference between the upstream side of the LNT13 and the downstream side of the DPF 14 is also provided.

Detection signals of these sensors are appropriately used for operationcontrol of the engine system 1000 by the ECU 200 including the equal-NOxcombustion control.

<Details of Equal-NOx Combustion Control>

Next, the contents of the equal-NOx combustion control of the enginesystem 1000 performed under the control of the ECU 200 in the presentembodiment will be described more specifically. FIG. 3 is a flowchartillustrating specific processing procedure of the equal-NOx combustioncontrol performed in the present embodiment. FIG. 4 is a flowchartillustrating a procedure of a combustion state calculation performed inthe equal-NOx combustion control. Furthermore, FIG. 5 is a graphillustrating a chronological change of the NOx concentration in a casethat combustion starts in the engine main body 100 under a certaintemperature (flame temperature) and still illustrating a chronologicalchange in a relationship (NOx generation characteristic) between the NOxconcentration in the exhaust gas EG from the engine main body 100 andflame temperatures from the start of combustion. In FIG. 5, a curve CVaindicates a chronological change of the NOx concentration when the flametemperature is Ta. Further, FIGS. 6A, 6B, and 6C are graphs illustratingcrank angle dependency of the cylinder temperature, the cylinderpressure, and the heat generation rate in the engine main body 100 inthe case of diffusion combustion.

In the equal-NOx combustion control, the NOx amount (NOx concentration)in the exhaust gas EG from the engine main body 100 is monitored whenthe vehicle equipped with the engine system 1000 is in the operatingstate. Such monitoring is performed by detecting the NOx in the exhaustgas EG at predetermined intervals by the NOx sensor NS1 provided nearthe upstream side of the LNT 13 (step S1). However, here, the normaloperation state is the subject of description. The content of equal-NOxcombustion during rich spike operation accompanied by post injection offuel performed to purge NOx from LNT 13 will be described later.

In the ECU 200, the amount of NOx detected by the NOx sensor NS1 iscompared with the reference value described in a reference NOx emissionamount map preliminarily held in the ECU 200 (step S2). The referenceNOx emission amount map is data obtained by mapping the reference valueof the amount of NO discharged from the engine main body 100 in thevehicle driving state with respect to the engine rotation speed (unit:rpm) and the torque (unit: N·m). In other words, it can be said that theECU 200 virtually constitutes determination means for determiningwhether or not the NOx amount in the exhaust gas falls within apredetermined reference range.

As long as the difference value between the detected NOx amount and thereference value described in the reference NOx emission amount map fallswithin the predetermined reference range (YES in step S2), suchmonitoring is only continued.

On the other hand, when the NOx amount detected by the NOx sensor NS1 isout of the reference range (NO in step S2), in the ECU 200, processingcalled a combustion state calculation, for estimating the combustionstate in the engine main body 100, more specifically, the crank angledependence of the heat generation (that is, the heat generation rateprofile), is executed (step S3). Therefore, it can be said that the ECU200 also virtually constitutes heat-generation-rate-profile estimationmeans for estimating the heat generation rate profile in the engine.

Briefly, the combustion state calculation is processing for estimatingthe heat generation rate profile as illustrated in FIG. 6C using theoutput of the NOx sensor NS1 and so forth, by utilizing the relationthat, when the cylinder temperature becomes maximum (maximum flametemperature) at the crank angle θ=θm (° CA) where a certain fuelinjection pulse ip2 is given, as illustrated in FIG. 6A, the cylinderpressure also becomes maximum at the crank angle θ=θm (° CA) asillustrated in FIG. 6B. It should be noted that, in FIG. 6A, FIG. 6B,and FIG. 6C, the crank angles of the intake valve closing timing IVC,the top dead center TDC of the piston, and the exhaust valve openingtiming EVO are defined as θα, θβ, and θγ, respectively. These are knownvalues determined according to the driving conditions of the vehicle.

In addition, FIG. 6C illustrates not only the estimated heat generationrate profile CV1 but also the heat generation percentage curve CV2corresponding to the cumulative frequency distribution thereof. Here,the points A to E (respective crank angles θa to θe) different from eachother in the crank angle marked in the heat generation rate profile CV1are respectively characterized as follows.

Point A: a point before the heat generation, in FIG. 6C, the point A isrepresented by the intake valve closing timing IVC, that is, θa=θα;

Point B: a point estimated to be a 10% heat generation point (estimated10% heat generation point);

Point C: a point estimated to be the maximum heat generation point(estimated maximum heat generation point);

Point D: a point estimated to end combustion (estimated combustion endpoint); and

Point E: a point after the heat generation, in FIG. 6C, the point E isrepresented by the exhaust valve opening timing EVO, that is, θe=θγ.

In the present embodiment, the crank angle at the point B, which is theestimated 10% heat generation point, is regarded as the ignition timingin the engine main body 100.

First, based on the output from the intake pressure sensor 24, thecylinder pressure at the point A (θ=θa=θα) which is also the intakevalve closing timing IVC is acquired (step S101).

Subsequently, by applying the output value from the NOx sensor NS1 tothe pre-specified NOx generation characteristic illustrated in FIG. 5,the flame temperature (combustion temperature) and the time to givethereof (that is, the crank angle θm) are obtained (Step S102).

The NOx generation characteristic illustrated in FIG. 5 does varyaccording to the elapsed time t immediately after the start ofcombustion, but it becomes substantially constant after approximately0.1 sec. has elapsed. For example, the functional relationship indicatedby the NO concentration-temperature curve CVb at the point of 0.1 sec.elapsed illustrated in FIG. 5 is to be established thereafter. NOx inthe exhaust gas EG to be detected by the NOx sensor NS1 reaches thearrangement position of the NOx sensor NS1 after some time (at least 0.1sec. or more) has elapsed from the generation, therefore, as long as thecombustion state in the engine main body 100 is held substantiallyconstant, the flame temperature can be specified by using the NOconcentration-temperature curve CVb.

In addition, from FIG. 5, the time at which the NOx concentration at theflame temperature is saturated after the start of combustion can bespecified as the time at which the generation of NOx is completed. Thetime at which generation of NOx is completed corresponds θ=θm in FIGS.6A, 6B, and 6C. For example, if the flame temperature is Ta and the NOxconcentration is n, corresponding to the point P on the NOconcentration-temperature curve CVb, the time at the point Q where theNOx concentration is saturated on the curve CVa gives θ=θm.

It is known that θ=θm substantially coincides with the 90% heatgeneration point in the heat generation percentage curve CV2. Therefore,by specifying θ=θm, the estimated 90% heat generation point isspecified. Further, it is also known that the central crank anglebetween the crank angle θm giving the 90% heat generation point and thecrank angle θβ of the top dead center TDC is equivalent to the crankangle θc of the point C (estimated maximum heat release rate point).Therefore, based on these relationships, the crank angle θc at theestimated maximum heat generation rate point (point C) is derived (stepS103).

The maximum heat generation rate point approximately coincides with the50% heat generation point, therefore, it can be said that the crankangle θc at point C gives the crank angle at the estimated 50% heatgeneration point.

Then, by linear extrapolation from the crank angle θm at the 90% heatgeneration point and the crank angle θc at the 50% heat generationpoint, the crank angle θb at point B estimated to be the 10% heatgeneration point can also be derived (step S104). As a result, theignition timing is estimated.

Subsequently, on the basis of the output from the exhaust pressuresensor 30, the cylinder pressure P0 at point E (θ=θe=θγ) which is alsothe exhaust valve opening timing EVO is acquired (step S105).

Finally, based on these series of processes, the heat generation rateprofile, that is, the heat generation history can be estimated in thecase where the NOx amount detected by the NOx sensor NS1 is out of thereference range (step S 106).

Briefly, the crank angles θb, θc, θm of the 10% heat generation point,the 50% heat generation point, and the 90% heat generation point arespecified in the heat generation percentage curve CV2, therefore, thefunction that gives the heat generation percentage curve CV2 can bederived. And, as a profile matching the obtained heat generationpercentage curve CV2, a function giving the heat generation rate profileCV1 is derived with the heat generation rate at the point C as the peakvalue (maximum heat generation rate). An appropriate simulation methodmay be applied to the specific calculation for such derivation. It hasbeen confirmed by the inventors of the present invention that the heatgeneration rate profile CV1 derived in such a manner substantially andfavorably coincides with the actual profile.

After the combustion state calculation is performed in the above mannerand the heat generation profile is estimated, subsequently, in the ECU200, a value of the index characterizing the heat generation profile iscompared with a value of the index in the ideal heat generation profiledetermined from the driving condition of the vehicle, and, based on thecomparison result, the combustion condition in the engine main body 100is controlled. This is one mode of the control operation of the ECU 200as the control means of the engine system 1000, which is the originalfunction thereof.

Specifically, first, the estimated ignition timing (crank angle θb atpoint B) is compared with the ideal ignition timing specified from thedriving conditions of the vehicle (step S4).

In the case that the difference value between the estimated ignitiontiming and the ideal ignition timing exceeds a predetermined referencerange (NO in step S4), it is determined whether the specified ignitiontiming is advanced (excessively advanced) or retarded (excessivelyretarded) with respect to the ideal ignition timing (step S5). Whenadvancing (excessively advanced), the operating condition is changed(step S6 a) so that the injection timing in the fuel injection pulse ip2can be retarded depending on the magnitude of excessiveness. Whenretarded (excessively retarded), the operating condition is changed(step S6 b) so that the injection timing in the fuel injection pulse ip2can be advancing depending on the magnitude of retardation.

Meanwhile, in the case that the difference value between the estimatedignition timing and the ideal ignition timing falls within thepredetermined reference range (YES in step S4), the maximum cylinderpressure estimated based on the estimated heat generation rate profileCV1 is either excessive or insufficient compared to the ideal maximumcylinder pressure specified from the driving condition of the vehicle(step S7). In either case, the difference value therebetween is out ofthe reference range.

When the maximum cylinder pressure is excessive, the boost pressure inthe VN turbo 11 is lowered or the opening of the valve VLV1 and/or VLV2is adjusted so that the recirculation amount (EGR amount) in the highpressure exhaust circulation device EGR1 and/or the low pressure exhaustcirculation device EGR2 can be increased (step S8 a). When the maximumcylinder pressure is insufficient, the boost pressure in the VN turbo 11is raised or the opening of the valve VLV1 and/or VLV2 is adjusted sothat the recirculation amount (EGR amount) in the high pressure exhaustcirculation device EGR1 and/or the low pressure exhaust circulationdevice EGR2 can be reduced (step S8 b).

After any of steps S6 a, S6 b, S8 a, and S8 b is performed, the processreturns to step S1 again to monitor NOx in the exhaust gas EG, and thecomparison in step S2 is performed. As a result, if the NOx amount (NOxconcentration) in the exhaust gas EG is still outside the referencerange, the processing from step S3 on down is repeated. For example, thefollowing may be the exemplified case, when the driving conditions ofthe vehicle are changed while the above procedure is being executed.

By executing the above procedure, the combustion condition is controlledsuch that the NOx amount (NOx concentration) in the exhaust gas EG ismaintained at a value commensurate with the combustion condition at thattime point. This is the equal-NOx combustion control during normaloperation in the present embodiment.

As described above, according to the present embodiment, the combustioncontrol of the engine can be suitably performed during normal operationwithout providing the cylinder pressure sensor. It should be noted that,although the NOx concentration detected by the NOx sensor has a largertime constant than the output value of the cylinder pressure sensor, thecontrollability in the equal-NOx combustion control according to thepresent embodiment is substantially equivalent to the controllability inthe combustion control using the cylinder pressure sensor, when takinginto consideration occurring of data thinning in the combustion controlusing the cylinder pressure sensor, delaying in control of intake andEGR system, and delaying in gas flow, and the like. As for constantlyperforming the combustion control every moment, the correction of thefuel injection timing using the cylinder pressure sensor is thequickest, however, the equal-NOx combustion control according to thepresent embodiment is supposed to perform correction when a deviationfrom an ideal combustion mode is detected at regular intervals,therefore, the value of the time constant of the output from the NOxsensor is not a particular disadvantage.

Also, the NOx sensor essential for the equal-NOx combustion controlaccording to the present embodiment is only the NOx sensor provided nearthe upstream side of the LNT 13, to which a NOx sensor provided forother purposes can be diverted, therefore, it is advantageous in termsof cost as compared with the case where an expensive cylinder pressuresensor is used.

Moreover, there is no influence of steps on the road surface at the timeof control, combustion control with high robustness against physicaldisturbance is executed as compared with the case where the cylinderpressure sensor that may be affected by the above factor is used.

<Equal-NOx Combustion Control During Rich Spike Operation>

Next, the equal-NOx combustion control during rich spike operation inwhich post injection is performed will be described. As is describedlater, the equal-NOx combustion control in such a case is characterizedin that it includes a step of determining the adequacy of the injectionamount and a step of making the ignition timing (post ignition timing)more appropriate for the post injection. FIG. 7 is a flowchartillustrating a specific processing procedure of equal-NOx combustioncontrol during rich spike operation. FIGS. 8A, 8B, and 8C are graphsillustrating crank angle dependency of a cylinder temperature, acylinder pressure, and a heat generation rate in the engine main body100 in the case of the rich spike operation.

When the vehicle on which the engine system 1000 is mounted shifts tothe rich spike operation for purging NOx from the LNT 13, the ECU 200calculates the air-fuel ratio (A/F) of the exhaust gas EG from theengine main body 100 (step S201). This calculation is performed based onthe output (the electromotive force value between predeterminedelectrodes) from the NOx sensor NS1 provided near the upstream side ofthe LNT 13. In other words, it can be said that the ECU 200 virtuallyconstitutes air-fuel ratio specifying means for specifying the A/F inthe exhaust gas EG.

The calculated value of A/F is compared with a reference rangepreviously determined based on the stoichiometric air-fuel ratio (stepS202). As long as the calculated value of A/F falls within the referencerange, such calculation is only repeated at predetermined intervals. Forexample, the range of 13.5 to 14.0 is preferably defined as thereference range.

Meanwhile, in the case that the calculated value of A/F is larger thanthe reference range, that is, in the case that the exhaust gas EG isexcessively lean, the fuel injection amount in the post injection isinsufficient, and accordingly NOx may be discharged from the LNT 13.Therefore, the NOx amount (NOx concentration) on the downstream side ofthe LNT 13 is obtained by the ECU 200. This is performed based on thedetection result of NOx by the NOx sensor NS2 provided on the downstreamside of the LNT 13 (step S203). In other words, it can be said that theECU 200 virtually constitutes downstream side NOx concentrationspecifying means.

When the amount of NOx detected by the NOx sensor NS2 is equal to orsmaller than a predetermined reference value (YES in step S204), noproblematic amount of NOx has been released from the LNT 13, and specialmeasures are unnecessary, therefore, the process returns to step S201again. It should be noted that, such a processing procedure is arelatively limited case such as when the LNT 13 is new.

On the other hand, when the amount of NOx detected by the NOx sensor NS2exceeds the predetermined reference value (NO in step S204), it meansthat the injection amount of fuel in the post injection is actuallyinsufficient; therefore, the post injection amount is increased (stepS205).

Meanwhile, in the case that the calculated value of A/F is out of thereference range and small, that is, in the case that the exhaust gas EGis excessively rich, there is a possibility that the fuel injectionamount in the post injection is merely excessive, yet there is apossibility that the combustion state in the engine main body 100 is notfavorable.

Therefore, in order to determine thereof, the NOx amount (NOxconcentration) on the upstream side of the LNT 13 is obtained by the ECU200. This is performed based on the detection result of NOx by the NOxsensor NS1 provided on the upstream side of the LNT 13 (step S206). Inother words, it can be said that the ECU 200 virtually constitutesupstream side NOx concentration specifying means.

When the amount of NOx detected by the NOx sensor NS1 is equal to orsmaller than the predetermined reference value (YES in step S207), itmeans that the fuel injection amount is actually excessive, so the postinjection amount is reduced (step S208).

Meanwhile, when the amount of NOx detected by the NOx sensor NS1 exceedsthe predetermined reference value (NO in step S207), the ECU 200executes the combustion state calculation (step S209) similarly to stepS3 (FIG. 3).

In the case of the rich spike operation, as illustrated in FIG. 8A, afuel injection pulse ip3 in which injection is added to the fuelinjection pulse ip2 during normal operation is given. In this case aswell, as in the case illustrated in FIG. 6A, assuming that the cylindertemperature becomes maximum (maximum flame temperature) at the crankangle θ=θm (° CA), the cylinder pressure also becomes the crank angleθ=θm (° CA). As is understood by comparing FIG. 6A with FIG. 8A andfurther between FIG. 6B and FIG. 8B, in the case of the rich spikeoperation, the profile shapes before θ=θm are the same except that asmall peak corresponding to post injection appears in the profiles ofthe cylinder temperature and the cylinder pressure.

Therefore, also in this case, by executing the combustion statecalculation in the procedure (steps S101 to S106) illustrated in FIG. 4,similarly to the above-described normal operation, the heat generationrate profile CV3 as indicated by the solid line in FIG. 8C, that is, theheat generation history is estimated (step S106). Here, the points A toE in the heat generation rate profile CV3 respectively correspond to thepoints A to E of the heat generation rate profile CV1 illustrated inFIG. 6C.

However, although the point E indicating the exhaust valve openingtiming EVO is defined as a point after heat generation (that is, a pointwhere the heat generation rate is 0) in the heat generation rate profileCV1 illustrated in FIG. 6C, in the heat generation rate profile CV3, theheat generation rate at the point E dose not drop 0 because the postinjection is performed. Thereafter, the cylinder pressure at the point Eacquired in the combustion state calculation in step S209 is set as P1.

In the present embodiment, suitability of ignition timing in postinjection is determined based on the comparison between the cylinderpressure P1 and the cylinder pressure P0 at the point E during normaloperation as described later. Specification of both the cylinderpressures P0 and P1 are performed as a part of the processing of thecombustion state calculation, those values are specified based on anoutput of the exhaust pressure sensor 30 at the exhaust valve openingtiming EVO, therefore, they can be specified independently of otherprocesses in the combustion state calculation performed to obtain theheat generation rate profile, in other words without performing otherprocesses. Therefore, when only the suitability of ignition timing inpost injection is focused, estimation of the heat generation rateprofile is eliminated.

Also, in the heat generation profile CV3 estimated in theabove-described manner, a peak corresponding to the post injectionappearing as the point F in the actual heat generation rate profile CV4illustrated in FIG. 8C does not appear. Nevertheless, the post injectionis performed in order to supply HC and CO as reducing agents forreducing NOx to cause incomplete combustion, and does not greatlycontribute to engine torque; therefore, the peak is not taken inconsideration in the equal NOx control.

After the combustion state calculation is performed and the heatgeneration rate profile CV3 is estimated, subsequently, a valuecharacterizing the heat generation rate profile CV3 is compared with apredetermined value in the ECU 200, and the combustion condition of theengine main body 100 during rich spike operation is controlled based onthe comparison result, This is also one mode of the control operation ofthe ECU 200 as the control means of the engine system 1000 which is theoriginal function thereof.

Specifically, first, the cylinder pressure P1 at the point E during richspike operation is compared with the cylinder pressure P0 at the point Eduring normal operation. And whether or not the ignition timing (postignition timing) in post injection satisfies the reference is determinedbased on whether or not the difference value P1−P0 therebetweensatisfies the predetermined reference range (step S210).

In the case that the difference value P1−P0 does not satisfy thereference range and therefore it is determined that the post ignitiontiming does not satisfy the reference range (NO in step S210), it isdetermined whether the ideal ignition timing is advanced (excessivelyadvancing) or retarded (excessively retarded) with respect to the idealignition timing (step S5). Specifically, in the case that the differencevalue P1−P0 does not satisfy the reference range and satisfies P1>P0, itis determined that the post injection timing is excessively advanced,while in the case that the difference value P1−P0 does not satisfy thereference range and satisfies P1<P0, it is determined that the postinjection timing is excessively retarded.

When the post ignition timing is excessively advanced, the operatingcondition is changed so that the post injection timing in the fuelinjection pulse ip3 is retarded in accordance with the advanced angle(step S212 a). When the post ignition timing is excessively retardedangle, the operating condition is changed so that the post injectiontiming in the fuel injection pulse ip3 is advanced in accordance withthe retarded angle (step S212 b).

Meanwhile, in the case that the difference value P1−P0 satisfies thereference range and therefore it is determined that the post ignitiontiming satisfies the reference range (YES in step S210), the maximumcylinder pressure estimated based on the estimated heat generation rateprofile CV3 is either excessive or insufficient compared to the idealmaximum cylinder pressure specified from the operating condition of thevehicle (step S213). In either case, the difference value therebetweenis out of the reference range.

In the case of excessive, the boost pressure in the VN turbo 11 islowered or the opening of the valve VLV1 and/or VLV2 is adjusted so thatthe recirculation amount (EGR amount) in the high pressure exhaustcirculation device EGR1 and/or the low pressure exhaust circulationdevice EGR2 can be increased (step S214 a). In the case of insufficient,the boost pressure in the VN turbo 11 is raised or the opening of thevalve VLV1 and/or VLV2 is adjusted so that the recirculation amount (EGRamount) in the high pressure exhaust circulation device EGR1 and/or thelow pressure exhaust circulation device EGR2 can be reduced (step S214a).

After any of steps S205, S208, S212 a, S212 b, S214 a, and S214 b isperformed, the process returns to step S201 again to monitor NOx in theexhaust gas EG, and the comparison in step S202 is performed. As aresult, if the NOx amount (NOx concentration) in the A/F is stilloutside the reference range, the processing from step S203 on down orstep S206 on down is repeated. For example, the following may be theexemplified case, for example, when the driving conditions of thevehicle are changed while the above procedure is being executed.

By executing the above procedure, the combustion condition for postinjection is controlled such that the A/F and the NOx amount (NOxconcentration) in the exhaust gas EG are maintained at a valuecommensurate with the rich spike operation condition at that time point.This is the equal-NOx combustion control during rich spike operation inthe present embodiment.

That is, according to the equal-NOx combustion control of the presentembodiment, in addition to the combustion control of the engine duringnormal operation, the equal-NOx combustion control during rich spikeoperation can be suitably performed.

In the equal-NOx combustion control according to the present embodiment,the output from the NOx sensor NS1 provided on the upstream side of theLNT 13 is used. On the other hand, in a conventional manner, the richspike operation is controlled based on the monitoring result of theoxygen concentration in the exhaust gas based on the output from theoxygen sensor OS provided on the downstream side of the LNT 13. Becausethe timing of detecting the enrichment is slow in this manner, it can besaid that the equal-NOx combustion control of the present invention issuperior to the conventional manner, in terms of real-time property andfuel economy.

<Modification of Equal-NOx Control During Normal Operation>

FIG. 9 is a flowchart illustrating of a modification of the processingprocedure of equal-NOx combustion control illustrated in FIG. 7. In themodification, as the premise of equal-NOx combustion control, the EGRrate when the vehicle on which the engine system 1000 is mounted is inthe operating state is monitored at predetermined intervals (step S301).The EGR rate is obtained as a ratio of recirculated exhaust air withrespect to the total intake air (a mixture of fresh air and recirculatedexhaust gas). Here, the fresh air intake amount is obtained from theflow rate of the fresh air specified by the air flow sensor 21.Meanwhile, the oxygen amount in the recirculated exhaust gas is obtainedfrom the oxygen concentration in the exhaust gas EG specified by the NOxsensor NS1 provided near the upstream side of the LNT 13, and the flowrate of the exhaust gas recirculated into the intake path 1 through thebranch paths 3 and 4, which is specified from the openings of the valvesVLV1 and VLV2. Also, the oxygen concentration in fresh air is knownbecause the oxygen concentration in atmospheric air coincides therewith.The oxygen concentration in the exhaust air is obtained from the oxygenconcentration output from the oxygen sensor OS and NOx sensor NSprovided on the exhaust system.

In the ECU 200, the obtained EGR rate is compared with the EGR rate inthe adaptation state described in the EGR map previously held in the ECU200 (step S302). Here, the EGR rate in the adaptation state is the valueof the EGR rate when the trade-off between the exhaust gas amount andfuel consumption is optimized. The EGR rate in the adaptation state isdetermined according to the torque and the engine rotational speed, andis stored in the ECU 200 in advance as the EGR map. In other words, itcan be said that the ECU 200 virtually constitutes determination meansfor determining whether or not the EGR rate falls within thepredetermined reference range.

As long as the obtained NOx amount and the difference value between theEGR rate in the adaptation state and the reference value falls withinthe predetermined reference range (YES in step S302), such monitoring isonly continued.

On the other hand, when the EGR rate is out of the reference range (NOin step S302), the NOx amount (NOx concentration) in the exhaust gas EGfrom the engine main body 100 is obtained. Such process is performed, asin step S1 (FIG. 3), by detecting the NOx in the exhaust gas EG by theNOx sensor NS1 provided near the upstream side of the LNT 13 (stepS303).

In the ECU 200, the NOx amount detected by the NOx sensor NS1 iscompared with the reference value described in the reference NOxemission amount map held in advance in the ECU 200 as in step S2 (FIG.3) (step S304).

When the difference value between the detected NOx amount and thereference value described in the reference NOx emission amount map fallswithin the predetermined reference range (YES in step S304), the EGRrate is monitored again. However, the detection of the NOx amount may berepeated.

On the other hand, when the NOx amount detected by the NOx sensor NS1 isout of the reference range (NO in step S304), the same processing asstep S5 on down in FIG. 3 is performed.

When the oxygen concentration in the exhaust gas changes, it isconsidered that the NOx concentration is also changing, and, in the caseof employing the NOx sensor NS having the configuration described later,the response of the sensor to oxygen is higher than the response of thatto NOx. When it is firstly determined whether or not the EGR ratesatisfies the reference range prior to the equal-NOx combustion controlto be performed, by setting the change of the oxygen concentration to acriterion, it is swiftly and advantageously determined whether or notthe equal-NOx combustion control can start execution.

Configuration Example of the NOx Sensor

FIG. 10 is a schematic cross-sectional view illustrating a configurationexample of the NOx sensor NS used in the engine system 1000.Hereinafter, regarding the configuration of the NOx sensor NS, a sensorelement N101 which is a main component thereof will be mainly described.However, the configuration of the NOx sensor NS used in the enginesystem 1000 is not limited to that illustrated in FIG. 10.

The sensor element N101, which is the main component of the NOx sensorNS, has a structure including a first substrate layer N1, an secondsubstrate layer N2, a third substrate layer N3, a first solidelectrolyte layer N4, a spacer layer N5, and a second solid electrolytelayer N6, each of which is formed of an oxygen ion conductive solidelectrolyte layer such as zirconia (ZrO₂), and the above layers arelaminated in this order from the lower side in the drawing. In addition,the solid electrolyte forming the six layers is dense and gas-tight.Such a sensor element N101 is produced, for example, by subjecting aceramic green sheet corresponding to each layer to predeterminedprocessing and printing of a circuit pattern, laminating thereof, andthen burning to integrate thereof.

Between the lower surface of the second solid electrolyte layer N6 andthe upper surface of the first solid electrolyte layer N4, which are oneend portion of the sensor element N101, a gas introduction port N10, afirst diffusion control part N11, a buffer space N12, a second diffusioncontrol part N13, a first internal space N20, a third diffusion controlpart N30, and a second internal space N40 are formed adjacent to eachother in such a manner as to communicate in this order.

The gas introduction port N10, the buffer space N12, the first internalspace N20, and the second internal space N40 are provided in such amanner that the spacer layer N5 is hollowed out, and are an internalspace in the sensor element N101 in which, the upper part thereof isdefined by the lower surface of the second solid electrolyte layer N6,the lower part thereof is defined by the upper surface of the firstelectrolyte layer N4, and a side part thereof is defined by the sidesurface of the spacer layer N5.

The first diffusion control part N11, the second diffusion control partN13, and the third diffusion control part N30 are all provided as twolaterally long slits (the opening has a longitudinal direction in adirection perpendicular to the drawing). The portion from the gasintroduction port N10 to the second internal space N40 is also referredto as a gas flow portion.

At a position farther from the end side than the gas flow portion, areference gas introduction space N43 is provided, the position of thereference gas introduction space N43 is defined between the uppersurface of the third substrate layer N3 and the lower surface of thespacer layer N5, and the side part thereof is defined by the sidesurface of the first solid electrolyte layer N4. For example,atmospheric air is introduced into the reference gas introduction spaceN43 as a reference gas for measuring the NOx concentration.

An atmosphere introduction layer N48 is a layer made of porous aluminaand a reference gas is introduced into the atmospheric introductionlayer N48 through the reference gas introduction space N43. Further, theatmospheric air introduction layer N48 is formed such that a referenceelectrode N42 is covered.

The reference electrode N42 is an electrode formed in a manner to beinterposed between the upper surface of the third substrate layer N3 andthe first solid electrolyte layer N4. As described above, theatmospheric air introduction layer N48 reaching the reference gasintroduction space N43, is provided in the surroundings of the referenceelectrode N42. In addition, as described later, by using the referenceelectrode N42, measurement of the oxygen concentration (oxygen partialpressure) in the first internal space N20 and the second internal spaceN40 can be performed.

In the gas flow portion, the gas introduction port N10 is a portionopened to the external space, and a measurement gas is taken into thesensor element N101 from the external space through the gas introductionport N10.

The first diffusion control part N11 is a part that imparts apredetermined diffusion resistance to the measurement gas taken from thegas introduction port N10.

The buffer space N12 is a space provided to introduce the measurementgas introduced from the first diffusion control part N11 to the seconddiffusion control part N13.

The second diffusion control part N13 is a part that imparts apredetermined diffusion resistance to the measurement gas introducedfrom the buffer space N12 into the first internal space N20.

When the measurement gas is introduced from the outside of the sensorelement N101 into the first internal space N20, the measurement gasrapidly taken into the sensor element N101 from the gas introductionport N10 by the pressure fluctuation of the measurement gas in theexternal space (pulsation of the exhaust pressure in the case where themeasurement gas is the exhaust gas of the automobile) is not directlyintroduced into the first internal space N20, but is introduced into thefirst internal space N20 after the concentration variation of themeasurement gas is canceled through the first diffusion control partN11, the buffer space N12, and the second diffusion control part N13. Asa result, the concentration fluctuation of the measurement gasintroduced into the first internal space N20 is mostly negligible.

The first internal space N20 is provided as a space for adjusting theoxygen partial pressure in the measurement gas introduced through thesecond diffusion control part N13. The oxygen partial pressure isadjusted by the operation of a main pump cell N21.

The main pump cell N21 is an electrochemical pump cell constituted of aninner pump electrode N22 having a ceiling electrode portion N22 aprovided on substantially the entire lower surface of the second solidelectrolyte layer N6 facing the first internal space N20, an outer pumpelectrode N23 provided in a manner of being exposed to an external spacein a region corresponding to the ceiling electrode portion N22 a, and asecond solid electrolyte layer N6 interposed between the electrodes.

The inner pump electrode N22 is formed such that the inner pumpelectrode N22 extends over the upper and lower solid electrolyte layers(the second solid electrolyte layer N6 and the first solid electrolytelayer N4) that define the first internal space N20 and the spacer layerN5 serving as a sidewall. Specifically, the ceiling electrode portionN22 a is formed on the lower surface of the second solid electrolytelayer N6 which serves as a ceiling surface of the first internal spaceN20, and the electrode portion N22 b is formed on the upper surface ofthe first solid electrolyte layer N4 which serves as a bottom surfacethereof. And, a side electrode portion (not shown) is formed on a sidewall surface (inner surface) of the spacer layer N5 constituting bothside wall portions of the first internal space N20 so as to connect theceiling electrode portion N22 a and the bottom electrode portion N22 b,and accordingly, the inner pump electrode N22 is disposed in a structurehaving a tunnel form at the disposition portion of the side electrodeportion.

The inner pump electrode N22 and the outer pump electrode N23 are formedas a porous cermet electrode (for example, a cermet electrode of Ptcontaining 1% Au and zirconia). In addition, the inner pump electrodeN22 in contact with the measurement gas is formed using a material whichis reduced in the reducing ability for the NOx component in themeasurement gas.

In the main pump cell N21, a desired pump voltage Vp0 is applied betweenthe inner pump electrode N22 and the outer pump electrode N23 to flow apump current Ip0 in the positive direction or the negative directionbetween the inner pump electrode N22 and the outer pump electrode N23,thereby oxygen in the first internal space N20 can be pumped out to theexternal space or oxygen in the external space can be pumped into thefirst internal space N20.

Also, in order to detect the oxygen concentration (oxygen partialpressure) in the atmosphere in the first internal space N20, the innerpump electrode N22, the second solid electrolyte layer N6, the spacerlayer N5, the first solid electrolyte layer N4, the third substratelayer N3, and the reference electrode N42 constitute an electrochemicalsensor cell, that is, a main pump-control oxygen-partial-pressuredetection sensor cell N80.

By measuring the electromotive force V0 in the main pump-controloxygen-partial-pressure detection sensor cell N80, the oxygenconcentration (oxygen partial pressure) in the first internal space N20can be determined. Further, the pump current Ip0 is controlled byfeedback-controlling Vp0 such that the electromotive force V0 isconstant. Accordingly, the oxygen concentration in the first internalspace N20 can be maintained at a predetermined constant value.

The third diffusion control part N30 applies a predetermined diffusionresistance to the measurement gas of which oxygen concentration (oxygenpartial pressure) is controlled by the operation of the main pump cellN21 in the first internal space N20 and leads the resultant gas to thesecond internal space N40.

The second internal space N40 is provided as a space for processing themeasurement of the nitrogen oxide (NOx) concentration in the measurementgas introduced through the third diffusion control part N30. Measurementof the NOx concentration is mainly performed in the second internalspace N40 of which oxygen concentration is adjusted by an auxiliary pumpcell N50, and then, further, operated by a measurement pump cell N41.

In the second internal space N40, the measurement gas introduced throughthe third diffusion control section N30 after the oxygen concentration(oxygen partial pressure) is adjusted in the first internal space N20 inadvance is further subjected to adjustment of the oxygen partialpressure by the auxiliary pump cell N50. As a result, the oxygenconcentration in the second internal space N40 can be kept constant withhigh accuracy, so that the NOx concentration can be measured with highaccuracy in the NOx sensor NS.

The auxiliary pump cell N50 is an electrochemical pump cell constitutedof an auxiliary pump electrode N51 having a ceiling electrode portionN51 a provided substantially over the entire lower surface of the secondsolid electrolyte layer N6 facing the second internal space N40, anouter pump electrode N23, (it is not limited to the outer pump electrodeN23 and it suffices if it is the sensor element N101 and any appropriateouter electrode), and the second solid electrolyte layer N6.

The auxiliary pump electrode N51 is disposed in the second internalspace N40 in a tunnel form similar to the inner pump electrode N22provided in the first internal space N20. That is, the ceiling electrodeportion N51 a is formed on the second solid electrolyte layer N6 whichserves as a ceiling surface of the second internal space N40, and theelectrode portion N51 b is formed on the first solid electrolyte layerN4 which serves as a bottom surface of the second internal space N40.And, each side electrode portion (not shown) is formed on the both wallsurfaces of the spacer layer N5 which serves as side walls of the secondinner space N40 so as to connect the ceiling electrode portion N51 a andthe bottom electrode portion N51 b, and accordingly, the auxiliary pumpelectrode N51 is made in a tunnel form.

Similarly to the inner pump electrode N22, the auxiliary pump electrodeN51 is also formed using a material which is reduced in the reducingability for the NOx component in the measurement gas.

In auxiliary pump cell N50, a desired pump voltage Vp1 is appliedbetween the auxiliary pump electrode N51 and the outer pump electrodeN23, thereby oxygen in the second internal space N40 can be pumped outto the external space or oxygen in the external space can be pumped intothe second internal space N40.

Also, in order to control the oxygen partial pressure in the atmospherein the second internal space N40, the auxiliary pump electrode N51, thereference electrode N42, the second solid electrolyte layer N6, thespacer layer N5, the first solid electrolyte layer N4, and the thirdsubstrate layer N3, constitute an electrochemical sensor cell, that is,an auxiliary pump-control oxygen-partial-pressure detection sensor cellN81.

With a variable power source N52 of which voltage is controlled based onthe electromotive force V1 detected in this auxiliary pump-controloxygen-partial-pressure detection sensor cell N81, the auxiliary pumpcell N50 performs pumping. Therefore, the oxygen partial pressure in theatmosphere in the second internal space N40 is controlled to a lowpartial pressure which does not affect measurement of NOx.

In addition, the pump current Ip1 is used together with the above tocontrol the electromotive force of the main pump-controloxygen-partial-pressure detection sensor cell N80. Specifically, thepump current Ip1 is input to the main pump-controloxygen-partial-pressure detection sensor cell N80 as a control signal,and the electromotive force V0 thereof is controlled so that thegradient of the oxygen partial pressure of the measurement gas to beintroduced from the diffusion control part N30 into the second internalspace N40 is controlled to be always constant. When used as a NOxsensor, the oxygen concentration in the second internal space N40 ismaintained at a constant value of about 0.001 ppm by the operation ofthe main pump cell N21 and the auxiliary pump cell N50.

The measurement pump cell N41 measures the NOx concentration in themeasurement gas in the second internal space N40. The measurement pumpcell N41 is an electrochemical pump cell constituted of a measurementelectrode N44 provided at a position on the upper surface of the firstsolid electrolyte layer N4 facing the second internal space N40 andspaced apart from the third diffusion control part N30, the outerpumping electrode N23, the second solid electrolyte layer N6, the spacerlayer N5, and the first solid electrolyte layer N4.

The measurement electrode N44 is a porous cermet electrode. Themeasurement electrode N44 also functions as a NOx a reducing catalystfor reducing NOx presenting in the atmosphere in the second internalspace N40. Further, the measurement electrode N44 is covered with afourth diffusion control part N45.

The fourth diffusion control part N45 is a film composed of a porousbody containing alumina (Al₂O₃) as a main component. The fourthdiffusion control part N45 serves to limit the NOx amount flowing intothe measurement electrode N44 and also functions as a protective film ofthe measurement electrode N44.

In the measurement pump cell N41, oxygen generated by decomposition ofnitrogen oxides in the atmosphere around the measurement electrode N44due to the catalytic activity of the measurement electrode N44 is pumpedout, and the generated amount can be detected as a pump current (alsoreferred to as NOx current) Ip2.

In order to detect oxygen partial pressure around the measurementelectrode N44, the second solid electrolyte layer N6, the spacer layerN5, the first solid electrolyte layer N4, the third substrate layer N3,the measurement electrode N44, and the reference electrode N42constitute an electrochemical sensor cell, that is, a measurementpump-control oxygen-partial-pressure detection sensor cell N82. Thevariable power source N46 is controlled based on the electromotive forceV2 detected by the measurement pump-control oxygen-partial-pressuredetecting sensor cell N82.

The measurement gas introduced into the second internal space N40reaches the measurement electrode N44 through the fourth diffusioncontrol part N45 under the condition that the oxygen partial pressure iscontrolled. The nitrogen oxides in the measurement gas around themeasurement electrode N44 are reduced to generate oxygen (2NO→N₂+O₂).The generated oxygen is pumped by the measurement pump cell N41. At thattime, the voltage Vp2 of a variable power source N46 is controlled sothat the electromotive force V2 detected by the measurement pump-controloxygen-partial-pressure detection sensor cell N82 becomes constant. Theamount of oxygen generating around the measurement electrode N44 isproportional to the concentration of nitrogen oxides in the measurementgas, therefore, the nitrogen oxide concentration in the measurement gasis calculated using the pump current Ip2 in the measurement pump cellN41.

Oxygen partial pressure detection means is constituted as anelectrochemical sensor cell by combining the measurement electrode N44,the first solid electrolyte layer N4, the third substrate layer N3, andthe reference electrode N42, thereby, a electromotive forcecorresponding to a difference between the amount of oxygen generated bythe reduction of the NOx component in the atmosphere around themeasurement electrode N44 and the amount of oxygen contained in thereference atmosphere can be detected, and this makes it possible toobtain the concentration of NOx component in the measurement gas.

Furthermore, the second solid electrolyte layer N6, the spacer layer N5,the first solid electrolyte layer N4, the third substrate layer N3, theouter pump electrode N23, and the reference electrode N42 constitute anelectrochemical sensor cell N83, and based on the electromotive forceV_(ref) obtained by the sensor cell N83, the oxygen concentration(oxygen partial pressure) in the measurement gas outside the sensor canbe specified. Specifically, the NOx sensor NS also has a function as anoxygen sensor.

In the NOx sensor NS having such a configuration, by operating the mainpump cell N21 and the auxiliary pump cell N50, the measurement gas ofwhich oxygen partial pressure is always kept at a constant low value (avalue substantially not affecting the measurement of NOx) is supplied tothe measurement pump cell N41. Therefore, based on the pump current Ip2flowing by pumping out oxygen generated by the reduction of NOx by themeasurement pump cell N41 substantially in proportion to theconcentration of NOx in the measurement gas, the concentration of NOx inthe measurement gas is grasped

More specifically, for each sensor element N101, a functionalrelationship (sensitivity characteristic) between the pump current Ip2and the NOx concentration is specified beforehand prior to use. Inactual detection of NOx, the value of Ip2 is continuously measured, andthe NOx concentration corresponding to each measurement value isobtained based on the previously specified sensitivity characteristic.

In addition, in the sensor element N101, by measuring the electromotiveforce V_(ref) generated between the outer pump electrode N23 and thereference electrode N42, the oxygen partial pressure outside the sensorelement N101 can be grasped.

In addition, the sensor element N101 includes a heater part N70 adoptedfor temperature adjustment for heating and retaining warmth of thesensor element N101 in order to increase oxygen ion conductivity of thesolid electrolyte. The heater part N70 includes a heater electrode N71,a heater N72, a through hole N73, a heater insulating layer N74, and apressure release hole N75.

The heater electrode N71 is an electrode formed in a manner in contactwith the lower surface of the first substrate layer N1. By connectingthe heater electrode N71 to an external power supply, electric power canbe supplied to the heater portion N70 from the outside.

The heater N72 is an electric resistor formed in a manner interposedbetween the second substrate layer N2 and the third substrate layer N3from above and below. The heater N72 is connected to the heaterelectrode N71 via the through hole N73, and generates heat by beingsupplied with the electric power from the outside through the heaterelectrode N71, thereby heating and retaining warmth of the solidelectrolyte forming the sensor element N101.

In addition, the heater N72 is embedded over the entire area from thefirst internal space N20 to the second internal space N40, so that theentire sensor element N101 can be adjusted to a temperature at which thesolid electrolyte is activated.

The heater insulating layer N74 is an insulating layer formed on theupper and lower surfaces of the heater N72 by an insulator such asalumina. The heater insulating layer N74 is formed to obtain electricalinsulation between the second substrate layer N2 and the heater N72 andelectrical insulation between the third substrate layer N3 and theheater N72.

The pressure release hole N75 penetrates through the third substratelayer N3 and is provided so as to communicate with the reference gasintroduction space N43. The pressure release hole N 75 is formed foralleviating an increase in internal pressure accompanying a temperaturerise in the heater insulating layer N74.

While the invention has been shown and described in detail, theforegoing description is in all aspects illustrative and notrestrictive. It is therefore understood that numerous modifications andvariations can be devised without departing from the scope of theinvention.

What is claimed is:
 1. A combustion control method in an engine of avehicle during a rich spike operation, wherein said engine includes afuel injection element attached to, and an exhaust path for an exhaustgas discharged from said engine includes a NOx occlusion catalyst and afirst NOx sensor provided on an upstream side of said NOx occlusioncatalyst, said method comprising the steps of: a) performing a richspike operation in said engine; b) specifying an air-fuel ratio of theexhaust gas discharged from said engine during said rich spike operationbased on a first output value output from said first NOx sensor inaccordance with an oxygen concentration in said exhaust gas; c)specifying a NOx concentration in said exhaust gas on the upstream sideof said NOx occlusion catalyst based on a second output value outputfrom said first NOx sensor in accordance with a NOx concentration insaid exhaust gas; and d) controlling a combustion condition of saidengine during said rich spike operation, wherein, in said step d), adifference value P1−P0 between a cylinder pressure P1 at an exhaustvalve opening timing in said engine during said rich spike operation anda cylinder pressure P0 at an exhaust valve opening timing in said engineduring a normal operation is calculated, when said air-fuel ratio isless than a predetermined reference range previously determined based ona stoichiometric air-fuel ratio and said NOx concentration exceeds apredetermined reference value, and in the case that said differencevalue P1−P0 does not satisfy the predetermined reference range andP1>P0, a fuel injection timing at a post injection from the fuelinjection element to said engine is restarted, and in the case that saiddifference value P1−P0 does not satisfy the predetermined referencerange and P1<P0, the fuel injection timing at the post injection fromsaid fuel injection element to said engine is advanced.
 2. Thecombustion control method according to claim 1 the method furthercomprising the step of e) estimating a heat generation rate profileindicating crank angle dependency of a heat generation rate in saidengine, wherein, in said step d), when said air-fuel ratio is less thanthe predetermined reference range previously determined based on thestoichiometric air-fuel ratio and said NOx concentration exceeds thepredetermined reference value, an estimated maximum cylinder pressureand an ideal maximum cylinder pressure are compared with each other, andthe combustion condition in said engine during said rich spike operationis controlled based on a result thereof, said estimated maximum cylinderpressure being estimated as a maximum cylinder pressure in said enginebased on said heat generation rate profile, and said ideal maximumcylinder pressure being a maximum cylinder pressure for said engine inthe ideal heat generation rate profile specified from a drivingcondition of said engine.
 3. The combustion control method according toclaim 2, wherein said engine includes a turbocharger, and in said stepd), the combustion condition in said engine during said rich spikeoperation is controlled by, lowering a boost pressure in saidturbocharger, in the case that said estimated maximum pressure is largerthan said ideal maximum pressure and a difference value therebetween isout of a predetermined reference range, and raising said boost pressure,in the case that said estimated maximum pressure is smaller than saidideal maximum pressure and a difference value therebetween is out of thepredetermined reference.
 4. The combustion control method according toclaim 2, wherein said engine includes an exhaust circulation elementattached thereto, in said engine, a portion of said exhaust gas isrecirculated by said exhaust circulation element to be re-sucked, and insaid step d), the combustion condition in said engine during said richspike operation is controlled by, increasing a recirculation amount ofsaid exhaust gas in said exhaust circulation element, in the case thatsaid estimated maximum pressure is larger than said ideal maximumpressure and a difference value therebetween is out of a predeterminedreference range, and reducing said recirculation amount, in the casethat said estimated maximum pressure is smaller than said ideal maximumpressure and a difference value therebetween is out of the predeterminedreference.
 5. The combustion control method according to claim 2,wherein, in said step e), each crank angle giving a 90% heat generationpoint, a 50% heat generation point, and a 10% heat generation pointwhich are corresponding to the heat generation rate profile isestimated, and then said heat generation rate profile is estimated basedon the corresponding obtained estimation results for said each crankangle, wherein each of the crank angles is estimated by specifying aflame temperature at NOx generation time, regarding said 90% heatgeneration point, specifying as a central point between said 90% heatgeneration point and a top dead center of said engine, regarding said50% heat generation point, and linear extrapolation from said 90% heatgeneration point and said 50% heat generation point, regarding said 10%heat generation point.
 6. The combustion control method according toclaim 1, wherein a second NOx sensor is provided on a downstream side ofsaid NOx occlusion catalyst in said exhaust path, said method furthercomprising said step of f) specifying a NOx concentration on thedownstream side of said NOx occlusion catalyst in said exhaust pathbased on a third output value output from said second NOx sensor inaccordance with the NOx concentration in said exhaust gas, when saidair-fuel ratio exceeds said predetermined reference range, wherein insaid step d), when the NOx concentration on the downstream side of saidNOx occlusion catalyst exceeds the predetermined reference value, a fuelinjection amount at said post injection is increased.
 7. The combustioncontrol method according to claim 1, wherein, in said step d), when saidair-fuel ratio is less than the predetermined reference range previouslydetermined based on the stoichiometric air-fuel ratio and said NOxconcentration on the upstream side exceeds the predetermined referencevalue, a fuel injection amount at said post injection is reduced.
 8. Anengine system for a vehicle including an engine and an exhaust pathincluding a NOx occlusion catalyst in the middle thereof wherein anexhaust gas from said engine is discharged outside through said exhaustpath, the system comprising: a first NOx sensor provided on an upstreamside of said NOx occlusion catalyst in said exhaust path; an air-fuelratio specification element configured to specify an air-fuel ratio ofsaid exhaust gas discharged from said engine during a rich spikeoperation based on a first output value output from said first NOxsensor in accordance with an oxygen concentration in said exhaust gas;an upstream side NOx concentration specification element configured tospecify a NOx concentration in said exhaust gas on the upstream side ofsaid NOx occlusion catalyst based on a second output value output fromsaid first NOx sensor in accordance with a NOx concentration in saidexhaust gas; and a rich spike combustion control element configured tocontrol a combustion condition of said engine during said rich spikeoperation, wherein the rich spike combustion control element beingconfigured to, calculate a difference value P1−P0 between a cylinderpressure P1 at an exhaust valve opening timing in said engine duringsaid rich spike operation and a cylinder pressure P0 at an exhaust valveopening timing in said engine during a normal operation, when saidair-fuel ratio is less than a predetermined reference range previouslydetermined based on a stoichiometric air-fuel ratio and said NOxconcentration exceeds a predetermined reference value, and in the casethat said difference value P1−P0 does not satisfy the predeterminedreference range and P1>P0, retard a fuel injection timing at a postinjection from a fuel injection element attached to said engine to saidengine, and in the case that said difference value P1−P0 does notsatisfy the predetermined reference range and P1<P0, advance a fuelinjection timing at a post injection from a fuel injection elementattached to said engine to said engine.
 9. The engine system for avehicle according to claim 8, further comprising: aheat-generation-rate-profile estimation element configured to estimate aheat generation rate profile indicating crank angle dependency of a heatgeneration rate in said engine, wherein, the rich spike combustioncontrol element is configured to compare an estimated maximum cylinderpressure with an ideal maximum cylinder pressure each other when saidair-fuel ratio is less than the predetermined reference range previouslydetermined based on said stoichiometric air-fuel ratio and said NOxconcentration exceeds the predetermined reference value, and control thecombustion condition in said engine during said rich spike operationbased on a result thereof, said estimated maximum cylinder pressurebeing estimated as a maximum cylinder pressure in said engine based onsaid heat generation rate profile, and said ideal maximum cylinderpressure being a maximum cylinder pressure for said engine in the idealheat generation rate profile specified from the driving condition ofsaid engine.
 10. The engine system for a vehicle according to claim 9,wherein said engine includes a turbocharger, and said rich spikecombustion control element is configured to control the combustioncondition in said engine during said rich spike operation by, lowering aboost pressure in said turbocharger, in the case that said estimatedmaximum pressure is larger than said ideal maximum pressure and adifference value therebetween is out of a predetermined reference range,and raising said boost pressure, in the case that said estimated maximumpressure is smaller than said ideal maximum pressure and a differencevalue therebetween is out of the predetermined reference.
 11. The enginesystem for a vehicle according to claim 9, wherein in said engine, aportion of said exhaust gas is recirculated by an exhaust circulationelement attached thereto to be re-sucked, and said rich spike combustioncontrol element is configured to control the combustion condition insaid engine during said rich spike operation by, increasing arecirculation amount of said exhaust gas in said exhaust circulationelement, in the case that said estimated maximum pressure is larger thansaid ideal maximum pressure and a difference value therebetween is outof a predetermined reference range, and reducing said recirculationamount, in the case that said estimated maximum pressure is smaller thansaid ideal maximum pressure and a difference value therebetween is outof the predetermined reference.
 12. The engine system for a vehicleaccording to claim 9, wherein said heat-generation-rate-profileestimation element is configured to estimate each crank angle giving a90% heat generation point, a 50% heat generation point, and a 10% heatgeneration point which are corresponding to said heat generation rateprofile, and then estimate said heat generation rate profile based onthe obtained estimation results for said each crank angle, wherein eachof the crank angles is estimated by specifying a flame temperature atNOx generation time, regarding said 90% heat generation point,specifying as a central point between said 90% heat generation point anda top dead center of said engine, regarding said 50% heat generationpoint, and linear extrapolation from said 90% heat generation point andsaid 50% heat generation point, regarding said 10% heat generationpoint.
 13. The engine system for a vehicle according claim 8, furthercomprising: a second NOx sensor provided on a downstream side of saidNOx occlusion catalyst in said exhaust path; and a downstream side NOxconcentration specification element configured to specify a NOxconcentration on the down stream side of said NOx occlusion catalyst insaid exhaust path based on a third output value output from said secondNOx sensor in accordance with the NOx concentration in said exhaust gas,when said air-fuel ratio exceeds said predetermined reference range,wherein said rich spike combustion control element is configured toincrease a fuel injection amount at said post, when the NOxconcentration on the downstream side of said NOx occlusion catalystexceeds the predetermined reference value.
 14. The engine system for avehicle according to claim 8, wherein the rich spike combustion controlelement is configured to reduce a fuel injection amount at the postinjection, when the air-fuel ratio is less than the predeterminedreference range previously determined based on the stoichiometricair-fuel ratio and the NOx concentration on the upstream side exceedsthe predetermined reference value.
 15. The combustion control methodaccording to claim 3, in said step e), each crank angle giving a 90%heat generation point, a 50% heat generation point, and a 10% heatgeneration point which are corresponding to the heat generation rateprofile is estimated, and then said heat generation rate profile isestimated based on the corresponding obtained estimation results forsaid each crank angle, wherein each of the crank angles is estimated byspecifying a flame temperature at NOx generation time, regarding said90% heat generation point, specifying as a central point between said90% heat generation point and a top dead center of said engine,regarding said 50% heat generation point, and linear extrapolation fromsaid 90% heat generation point and said 50% heat generation point,regarding said 10% heat generation point.
 16. The combustion controlmethod according to claim 2, wherein a second NOx sensor is provided ona downstream side of said NOx occlusion catalyst in said exhaust path,said method further comprising said step of f) specifying a NOxconcentration on the downstream side of said NOx occlusion catalyst insaid exhaust path based on a third output value output from said secondNOx sensor in accordance with the NOx concentration in said exhaust gas,when said air-fuel ratio exceeds said predetermined reference range,wherein in said step d), when the NOx concentration on the downstreamside of said NOx occlusion catalyst exceeds the predetermined referencevalue, a fuel injection amount at said post injection is increased. 17.The combustion control method according to claim 2, wherein, in saidstep d), when said air-fuel ratio is less than the predeterminedreference range previously determined based on the stoichiometricair-fuel ratio and said NOx concentration on the upstream side exceedsthe predetermined reference value, a fuel injection amount at said postinjection is reduced.
 18. The engine system for a vehicle according toclaim 10, said heat-generation-rate-profile estimation element isconfigured to estimate each crank angle giving a 90% heat generationpoint, a 50% heat generation point, and a 10% heat generation pointwhich are corresponding to said heat generation rate profile, and thenestimate said heat generation rate based on the obtained estimationresults for said each crank angle, wherein each of the crank angles isestimated by specifying a flame temperature at NOx generation time,regarding said 90% heat generation point, specifying as a central pointbetween said 90% heat generation point and a top dead center of saidengine, regarding said 50% heat generation point, and linearextrapolation from said 90% heat generation point and said 50% heatgeneration point, regarding said 10% heat generation point.
 19. Theengine system for a vehicle according to claim 9, further comprising: asecond NOx sensor provided on a downstream side of said NOx occlusioncatalyst in said exhaust path; a downstream side NOx concentrationspecification element configured to specify a NOx concentration on thedown stream side of said NOx occlusion catalyst in said exhaust pathbased on a third output value output from said second NOx sensor inaccordance with the NOx concentration in said exhaust gas, when saidair-fuel ratio exceeds said predetermined reference range, wherein saidrich spike combustion control element is configured to, when the NOxconcentration on the downstream side of said NOx occlusion catalystexceeds the predetermined reference value, increase a fuel injectionamount at said post.
 20. The engine system for a vehicle according toclaim 9, wherein the rich spike combustion control element is configuredto reduce a fuel injection amount at the post injection, when theair-fuel ratio is less than the predetermined reference range previouslydetermined based on the stoichiometric air-fuel ratio and the NOxconcentration on the upstream side exceeds the predetermined referencevalue.